Among the most common designs of heat exchangers in operation is the tube and tubesheet type in which tubes extend between a pair of tubesheets and contain a flowing fluid which exchanges heat with a second fluid flowing between the tubesheets and over the tubes. In such a design, it is important that the tubes, which, at their end portions extend through holes in the tubesheets are welded so that there is no leakage through the tubesheet holes. The tubes are conventionally welded at their ends to the tubesheets at the tubesheet surfaces which are closest to the ends of the tubes.
The tubes are conventionally positioned so that their ends project a predetermined distance beyond the tubesheets by a procedure commonly referred to as staking and setting.
The tubes are stacked by placing a prick punch against the face of the tubesheet with the working end adjacent to the tube to be set. The punch is struck with a hammer to move a small amount of metal against the tube. This operation is usually performed at several locations about the tube hole, perhaps at three points spaced 120 degress apart.
The above described staking and setting operations are not without their drawbacks. For one, they are time consuming. Quite often, the tube will slip back into the tubesheet hole requiring subsequent retrieval with a special tool followed by restaking and resetting.
The tube wall is sometimes deformed during the staking operation. This is usually due to the punch being held at the wrong angle, causing it to glance off the tubesheet and into the tube wall. As a result, dimples on the outer tube surfaces are formed which have to be removed prior to welding because they interfere with the operation of the welding equipment which is used to join the tubes to the tubesheets.
After the staking operation, the heat exchanger is often moved, as for example, by a crane, to a location where the tubes are permanently welded in place. During this transfer, the tubes often flex causing some of them to pull loose, thus requiring subsequent restaking and resetting.
Often, after several of the tubes are permanently welded, the operator must restake adjacent tubes because the heat generated by welding tends to loosen them.
Thus the staking and setting operations often have to be performed several times during the manufacture of a tube to tubesheet heat exchanger. Therefore, the operations necessary to properly position the tubes so that they can be permanently welded to the tubesheet are lengthly and therefore expensive.